This invention relates to apparatus for protecting aquatic life passing through a water filter and more particularly to means for protecting aquatic life carried by water passing through a screening unit embodying an endless, vertical traveling band screen formed of a plurality of screening baskets which open inwardly and pass around upper and lower terminal members with the water flowing through each screening basket from the inner side toward the outer side thereof.
As is well known in the art to which our invention relates, difficulties have been encountered in protecting aquatic life passing through filter systems, such as the cooling-water intake structure of a power plant. As pointed out in the January, 1977 issue of "POWER" many devices have been proposed for protecting aquatic life at power plant intakes. There are disadvantages in prior art methods of protecting aquatic life due to the fact that the aquatic life is not handled in a gentle manner whereby the aquatic life is tranferred, without injury, from the filter system back to the source water. Potential damage to the aquatic life may be caused by either impingement or entrainment. Impingement occurs when aquatic organisms are trapped against an intake screen by the force of the intake velocity across the screen. The effects of impingement most common on young fish include starvation and exhaustion, asphyxiation, and descaling due to screen spray water. Impingement also applies to entrapped organisms which are drawn into the vicinity of the intake or discharge structure.
Entrainment relates to the transport of organisms small enough to pass through the screen mesh, which subsequently are exposed to abrasion within pumps and condenser tubing, pressure changes through the system, thermal shock in the condenser or discharge tunnel, and exposure to chemical antifoulants. Fish eggs and larvae are the organisms most commonly entrained due to the fact that they have little motile ability and usually cannot escape or avoid the intake flow.
While fish bucket systems have been proposed, such as disclosed on pages 74 and 76 of the January, 1977 "POWER" publication, such fish buckets open outwardly and are rotated almost 180.degree. before the fish are discharged and at the same time the fish fall a substantial distance within the bucket just prior to striking a flexible deflector which directs the fish into a sluice trough.
Our improved apparatus for protecting aquatic life may be employed with traveling band screens of the general type shown in the Jopp U.S. Pat. No. 4,040,952 having inwardly opening screening baskets which are generally semi-cylindrical, as viewed in cross section. Also, the screening baskets may assume other shapes, such as that shown in the Passavant U.S. Pat. No. 3,458,046.